In recent years, development of clean energy is desired due to the problem of exhaustion of energy resources, the global environmental problem such as increase in CO2 in the atmosphere, and the like, and photovoltaic generation particularly utilizing solar cells among photoelectric conversion elements is being developed, practically applied, and progressed as a new energy source.
A typical solar cell is a bifacial solar cell fabricated by diffusing impurities of a conductivity type opposite to that of a monocrystalline or polycrystalline silicon substrate, for example, into a light-receiving surface of the silicon substrate to form a pn-junction, and forming electrodes at the light-receiving surface and the back surface opposite to the light-receiving surface, respectively. In the bifacial solar cell, it is also common to diffuse impurities of the same conductivity type as that of the silicon substrate, into the back surface of the silicon substrate at high concentration, thereby increasing outputs by a back surface field effect.
Research and development is also being made on a back electrode type solar cell with no electrode formed at a light-receiving surface of a silicon substrate but with an electrode formed only at the back surface (see, e.g., Pamphlet of WO2007/081510 (Patent Literature 1)).
Referring to schematic sectional views of FIG. 7(a) and g. 7(b), a method of fabricating a back electrode type solar cell described in Patent Literature 1 will be described below.
First, as shown in FIG. 7(a), a low-concentration n-type dopant source 101, a high-concentration n-type dopant source 102, a low-concentration p-type dopant source 103, and a high-concentration p-type dopant source 104 are formed by inkjet printing or screen printing at the back surface of a silicon substrate 100 which is a surface opposite to the side where a textured structure 108 is formed.
Then, as shown in FIG. 7 (b), silicon substrate 100 is heat-treated to diffuse an n-type dopant at low concentration from low-concentration n-type dopant source 101 to form a low-concentration n-type-dopant diffusion layer 116 in the back surface of silicon substrate 100, and diffuse an n-type dopant at high concentration from high-concentration n-type dopant source 102 to form a high-concentration n-type-dopant diffusion layer 105. Further, a p-type dopant is diffused at low concentration from low-concentration p-type dopant source 103 to form a low-concentration p-type-dopant diffusion layer 115, and a p-type dopant is diffused at high concentration from high-concentration p-type dopant source 104 to form a high-concentration p-type-dopant diffusion layer 106.
Latest researches have revealed that a back electrode type solar cell of high characteristics is obtained when a low-concentration dopant diffusion layer such as low-concentration n-type-dopant diffusion layer 116 and low-concentration p-type-dopant diffusion layer 115 are formed between high-concentration dopant diffusion layers of different conductivity types such as high-concentration n-type-dopant diffusion layer 105 and high-concentration p-type-dopant diffusion layer 106 in the back surface of silicon substrate 100, as shown in FIG. 7(b).
However, in the method disclosed in Patent Literature 1 described above, silicon substrate 100 is heat-treated after low-concentration n-type dopant source 101, high-concentration n-type dopant source 102, low-concentration p-type dopant source 103, and high-concentration p-type dopant source 104 are formed by inkjet printing or screen printing.
Therefore, the method disclosed in Patent Literature 1 described above is disadvantageous in that diffusion of dopants in the back surface of the silicon substrate is not controllable since the dopants out-diffuse from the above-described dopant sources during heat treatment of silicon substrate 100, causing the dopants of different conductivity types to diffuse mutually in the gas phase surrounding silicon substrate 100.
Therefore, to prevent out diffusion of dopants from dopant sources, a method of forming a mask on the dopant sources and then heat-treating is proposed (see, e.g., Japanese Patent Laying-Open No. 2008-78665 (Patent Literature 2)).
Referring to schematic sectional views of FIG. 8(a) to FIG. 8(e), a method of fabricating a back electrode type solar cell disclosed in Patent Literature 2 will be described below.
First, as shown in FIG. 8(a), a textured structure 208 is formed at one surface of a silicon substrate 200.
Then, as shown in FIG. 8(b), an oxide layer 209 is formed on textured structure 208 at the surface of silicon substrate 200.
Then, as shown in FIG. 8(c), a p-type doping paste 203 containing a p-type dopant and an n-type doping paste 204 containing an n-type dopant are applied at a predetermined spacing at the back surface of silicon substrate 200 of p- or n-conductivity type which is the surface opposite to the side where textured structure 208 is formed.
Then, as shown in FIG. 8(d), an oxide layer 202 is formed to cover p-type doping paste 203 and n-type doping paste 204 at the back surface of silicon substrate 200.
Thereafter, silicon substrate 200 is heat-treated to diffuse the p-type dopant from p-type doping paste 203 and diffuse the n-type dopant from n-type doping paste 204 into the back surface of silicon substrate 200. A p-type-dopant diffusion layer 205 and an n-type-dopant diffusion layer 206 are thereby formed, respectively, in the back surface of silicon substrate 200, as shown in FIG. 8(e).
Then, a metallized portion 210 is formed on p-type-dopant diffusion layer 205 in the back surface of the silicon substrate 200, and a metallized portion 211 is formed on n-type-dopant diffusion layer 206, thereby fabricating the back electrode type solar cell disclosed in Patent Literature 2.